1. Field of the Invention
The present invention relates to an apparatus for polishing substrates. More particularly, the invention relates to a platen/polishing pad assembly having a compliant surface to improve polishing uniformity of substrates.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Often it is necessary to polish a surface of a substrate to improve substrate surface uniformity by removing high topography, surface defects, scratches or embedded particles as well as improve polishing consistency from one substrate to the next to form substrates with consistent substrate surfaces for subsequent polishing. One common polishing process is known as chemical mechanical polishing (CMP) and is used to improve the quality and reliability of the electronic devices formed on the substrate.
Typically, the polishing process involves the introduction of a chemical slurry during the polishing process to facilitate chemical reaction with material components of the substrate surface to provide for higher removal of material components therefrom. In general, the polishing process involves holding a substrate against a polishing pad under controlled pressure, temperature and relative speed (velocity) of the pad in the presence of the slurry or other fluid medium. One polishing system that is used to perform CMP is the Mirra® CMP System available from Applied Materials, Inc., and shown and described in U.S. Pat. No. 5,738,574, entitled, “Continuous Processing System for Chemical Mechanical Polishing,” the entirety of which is incorporated herein by reference.
An important goal of CMP is achieving uniform substrate and die (substrate to substrate) planarity of substrate surfaces. Uniform planarity includes the uniform removal of material from across the surface of substrates as well as removing topographical defects and layer variations that have been deposited on the substrate. Further, uniformity must be achieved not only for a single substrate, but also for a series of substrates processed in a batch for consistent substrate to substrate processing.
One factor that can detrimentally affect polishing uniformity of substrates during sequential substrate polishing is cross-contamination. One source of cross-contamination are particles on the polishing pad that may be generated from polishing processes performed in the polishing system. Particles in the polishing system can originate from materials 10, such as tungsten and copper, deposited on the edge area 40 of a substrate 1 as shown in FIG. 1. For example, one source of particles in the polishing system can arise from material 10 deposited on the beveled edge 20 of a substrate 1, which can flake or peel off during chemical mechanical polishing.
Another potential problem is the excess amount of material that may be deposited on an edge bead removal (EBR) area 30 of the substrate. The excess material may be dislodged as particles from the substrate during polishing to form a source of cross-contamination. Excess materials dislodged from the EBR area 30 may be dislodged by a tearing phenomena and remove material from adjacent portions of the substrate or devices formed on the substrate. Additionally, the material deposited on the beveled edge 20 and the EBR area 30 is usually of a different level as the material deposited on the rest of the substrate surface, which may not be sufficiently planarized with the rest of the substrate surface when polished and result in a non-uniform surface.
Currently, polishing processes remove tungsten using a “hard” or “soft” polishing pad. Hard polishing pads, or rigid polishing pads, have a higher removal rate of material and longer processing life than soft pads, but hard polishing pads 50 generally have difficulty removing material from the bevel edge 20 of a substrate 1 as shown in FIG. 2. Examples of hard polishing pads are the IC-1000 and IC-1010 polishing pads commercially available from Rodel Inc. of Phoenix Ariz. (IC-1000 is a product name of Rodel, Inc.)
Soft polishing pads, or compliant polishing pads, have the ability to flex to low bevel areas 20 and remove deposited materials from the edge area 40 of the substrate, but often have a lower removal rate, decreased uniform polishing, and a shorter processing life than hard polishing pads due to their weaker mechanical properties. Additionally, soft polishing pads 60 may overpolish materials 70 in a substrate feature to form a topographical defect referred to as dishing as shown in FIG. 3. An example of a soft polishing pad is the Politex or Suba IV commercially available from Rodel Inc. of Phoenix, Ariz. (Politex and Suba IV are product names of Rodel, Inc.) The non-planar nature of dishing can detrimentally affect subsequent processing of the substrate surface.
One proposed solution to remove material from the bevel edge 20 and EBR area 30 is to first polish the substrate with a hard pad to remove the majority of conductive material from the substrate surface and then polishing the substrate with a softer pad to buff the substrate remove remaining conductive material from the bevel edge 20 and any necessary dielectric material. Buffing involves polishing the substrate on a platen with a low application of force between the substrate surface and the polishing pad and generally using a polishing composition with a low material removal rate compared to conventional polishing processes.
However, the use of two separate pads for bevel edge 20 and EBR area 30 removal increases the number of processing steps and processing time, decreases substrate through-put, increases operating costs, and increases equipment usage. Additionally, the hard pad may still dislodge the excess deposition at the edge area 40 of the substrate that can damage adjacent portions of the substrate prior to polishing with the soft pad, and the soft polishing pad may still result in dishing or overpolishing of the substrate features.
Another solution to remove material from the bevel edge 20 and EBR area 30 is to use a stack of polishing pads, or a composite pad, such as a hard polishing pad disposed on a soft polishing pad. The combination of pads having the proper proportions of hardness and compliance, or flexibility, is believed to achieve good planarity and uniformity over the surface of the substrate as well as improve polish of the edge area of the substrate.
However, a number of problems are associated with composite pads. One problem with composite pads is the interdependence of the individual pads upon one another. For example, a pressure exerted on the upper pad is transmitted to the lower pad. Because the upper pad is generally a rigid material having limited compressibility, applied pressure is accommodated by deformation of the upper material, or more often, by displacement of the position of the upper pad. The displacement of the upper material results in pressure being applied to the lower pad. The pressure on the lower pad is partially absorbed by compression of the lower pad and can result in deformation of the lower pad.
In the case of a shearing force, such deformation can result in ripples or waves on the lower pad due to the mass compression and redistribution of the lower pad which, during operation, exert a resultant force on the upper pad which can result in non-uniform polishing and undermine the goal of substrate planarization. Efforts to prevent pressure deflection and improve planarity by the use of lower pads made of composite materials have been less than successful in preventing pressure deflection and are not sufficiently effective in removing material from the bevel edge 20 and EBR area 30 of the substrate.
Additionally, since the upper pad of a composite pad includes a polishing surface of a uniform hardness and an underlying layer of softer material to establish a desired flexibility of the composite pad, the flexibility of the lower pad rarely translates into flexibility of the upper surface in relationship to polishing substrate surfaces that are not planar to the polishing pad. For example, the upper pad may still lack the require flexibility of the lower pad to remove substrate materials from the bevel edge 20 of a substrate 1 as shown in FIG. 2.
Another problem with composite pads is that each additional layer, e.g., pad and adhesive layer, in the composite pad acts as a source of variation affecting the overall stiffness, compression and/or compliance of the composite pad. The greater the number of layers or even variations in the thickness of pads, the greater the potential for variation. As a result, a polishing device utilizing a composite pad is often unable to achieve desired polishing results over a number of substrates. In addition, the planarity may change as the upper pad wears away by a process known as conditioning the pad. As the upper pad is reduced in thickness, the planarity may decrease with increasing numbers of substrates polished on the pad.
One solution to the difficulty of composite pads of multiple layers is to polish substrates with a single layer pad having both hard and soft materials. One example of such pads are polishing pads having concentric rings of polishing materials with the ring being composed of materials having different hardness. For example, in U.S. Pat. No. 5,944,583, ('583), a polishing pad is disclosed having alternating concentric rings of hard and soft materials. However, the '583 case uses the rings of polishing materials to provide consistent and uniform coverage of polishing slurry across the substrate surface during polishing and does not disclose polishing of excess material from the from the bevel edge 20 of a substrate 1 as shown in FIG. 2. Additionally, the '583 polishing pad requires polishing the substrate surface on the multiple polishing areas, which are substantially less than the diameter of the wafer, and does not disclose selective polishing of portions of a substrate surface, such as the bevel edge of a substrate.
U.S. Pat. No. 6,168,508 discloses a polishing pad multiple polishing areas having different properties, such as hardness, for polishing a substrate to derive the benefits from polishing with both hard and soft materials. However, the polishing pad is designed for polishing a substantial portion of the substrate surface on the multiple polishing areas, which are substantially less than the diameter of the wafer. Additionally, the multiple polishing areas do not provide adequate or selective edge polishing and removal of excess material from the from the bevel edge 20 of a substrate surface.
Therefore, there is a need for an article of manufacture, process, and apparatus for polishing the edge of a substrate.